The present invention relates to improvements in apparatus for ascertaining the pressure in a plenum chamber. More particularly, the invention relates to improvements in apparatus for ascertaining the pressure in a plenum chamber without the need for penetrating into the plenum chamber.
The human eyeball can be said to constitute a plenum chamber, i.e., an object having an envelope surrounding an internal space which is maintained above atmospheric pressure. Apparatus for ascertaining the tension of eyeballs are known as tonometers, and the following description will refer primarily to tonometers with the understanding, however, that the apparatus of the present invention can be practiced with equal or similar advantage to ascertain the pressure in other types of plenum chambers wherein the shell or envelope for a body of fluid comprises a flexible portion.
The need for apparatus or instruments which can be used to ascertain the tension of an eyeball without puncturing the eyeball or drilling a hole therein is particularly pronounced in certain branches of ophthalmology. For example, timely treatment of glaucoma (defined as a disease of the eye marked by increased pressure within the eyeball that damages the optic disk and results in gradual loss of vision and ultimate blindness) necessitates the provision of means for measuring the tension of the eyeball. The pressure in a healthy human eyeball is approximately 26.66 mbar (20 mm Hg). The presently known tonometers which are used for such purposes are quite complex, bulky and expensive and, as a rule, can be operated properly only by physicians or by other specially and highly trained persons.
The article on pages 1791/1793 of the November 1983 volume (No. 101) of the publication entitled "Arch Ophtalmol" describes a tonometer which can be manipulated by a patient. In contrast to an electronic tonometer or an electronic tonograph (which is manipulated by a physician or by a highly skilled and trained person and serves to ascertain the magnitude of force which is needed for applanation of the cornea), the tonometer which is disclosed in the aforementioned publication is designed to measure the pressure which is needed to carry out an applanation of the cornea. In either event, it is necessary to reduce the sensitivity of the cornea by administering a pain killer and/or by the application of a thin contact lens.